Project Summary/Abstract Autism Spectrum Disorder (ASD) is a neurodevelopmental disease affecting almost 2% of children in the US alone. Despite its genetic and clinical heterogeneity, recent systems biology and genomics studies demonstrated that ASD converges on a specific set of cellular pathways. Epigenetic regulation and synaptic signaling emerged as the two most prominent pathways in ASD, with many high-confidence genetic risk factors and dysregulated genes involved in these processes. This observation prompted a hypothesis that epigenetic dysregulation leads to improper neuronal circuit development and function, which has been demonstrated in mouse models of epigenetic regulators recurrently mutated in ASD, such as CHD8 (Chromodomain Helicase DNA Binding Protein 8). However, the exact epigenetic changes, cell types they affect and the neuronal circuitry changes resulting from epigenetic dysregulation in ASD are unknown. Recently, single-cell genomics approaches, including single- cell RNA sequencing and single-cell ATAC sequinning, offered unprecedented new level of detail of cellular and molecular composition of the brain, as well as processes underlying its development. In my postdoc, I applied single-nucleus RNA sequencing to human post-mortem cortical tissue from ASD patients to gain insight into the molecular changes associated with ASD in specific neuronal and glial subtypes. One of the most important insights from this work is the implication of upper-layer cortical neurons as the cell type most affected by ASD- associated transcriptional changes. This observation raises questions about the origin and functional effects of such changes on specific neuronal circuits. As part of the Aim 1 of my K99 proposal, I will test the hypothesis that gene expression changes in ASD are driven by changes in epigenetic states of specific cell types. To that end, I will perform a joint RNA-seq and ATAC-seq profiling of neocortical tissue of ASD patients and controls to identify cell type-specific epigenetic changes. Then, I will develop and test a high-throughput synaptic tracing technique by combining barcoded rabies virus library with single-nucleus RNA sequencing (Aim 2 of K99 phase). Finally, using the training, tools and preliminary data from the K99 phase of my proposal, I will launch an independent research project that focuses on investigating cell-type specific epigenetic and neuronal circuitry changes in the Chd8+/ mouse model during development (R00 phase). I will first apply the joint RNA-seq/ATAC- seq protocol to study epigenetic changes in specific cell types during development caused by the loss of one of Chd8 alleles. By crossing the Chd8+/ mouse with reporter lines expressing Cre recombinase in specific neuronal subtypes, such as upper-layer cortical neurons (Cux2-Cre), I will be able to use the barcoded rabies virus library and single-nucleus RNA-seq to identify changes in specific components of cortical circuitry as the result of Chd8 haploinsufficiency. I believe that the K99-R00 award will allow me to form a unique research direction and establish myself as a successful independent investigator in the area of autism and single-cell genomics.